The peroxidase test is the industry default check that a vegetable has been adequately
blanched. It is a rapid colour reaction run at the line: if peroxidase activity is
present, the reagent changes colour.
The logic is indirect. Peroxidase is one of the most heat-resistant enzymes in
vegetable tissue, so if it has been inactivated, the less heat-stable enzymes that
actually cause most quality defects should have been inactivated too. The test is
cheap, fast and gives a visible result, which is why it has survived for decades.
It has two documented weaknesses. Partially denatured peroxidase can regenerate
activity during storage, so a negative result immediately after blanching does not
always stay negative – this matters most where the blanch was marginal. And because
peroxidase is more heat-resistant than the enzymes causing the defects, blanching to
full peroxidase inactivation over-blanches relative to what quality alone would
require, costing texture and nutrients. Some processors validate against lipoxygenase
instead, or alongside it.
The important distinction for a buyer is between validation and verification. The
blanch should be validated as a process – time, temperature, loading, equipment – and
the enzyme test used routinely to verify that the validated process is being delivered.
A plant that has only the test, and no validation behind it, has a check rather than a
control.